Kozai cycles as possible trigger mechanism for producing short-period
binaries and other exotic objects

L. Kisseleva-Eggleton, P. Eggleton
California State University Maritime Academy (USA)

When three point masses form a hierarchical triple system, the
short-period orbit can be severely modified by the long-period orbit,
especially if the two orbits are inclined to each other by more than
about 39$^{\circ}$. Such an inclination can induce `Kozai
cyclesÕ, in which the eccentricity of the inner orbit cycles by a
large amount while its period and therefore semimajor axis remains
roughly constant. During those periastra when the eccentricity is
largest, tidal friction may become important, and this can result in a
secular shrinkage of the orbit, until it becomes circularised at
a period of a few days. Kozai cycles are not an unbeatable way of
bringing stars closer together. Apsidal motion due to either GR or to
the quadrupolar distortion of the
components (if they are no longer treated as point masses) can prevent
the cycles from reaching the extremes of eccentricity high enough to
allow tidal friction to operate efficiently. We explore the limits on
outer and inner orbital period that these perturbations imply. If the
components are F/G/K/M dwarfs, then rotationally-driven dynamo activity
can become important at the short periods that can occur in the right
circumstances. It can cause the period to shorten further. The result
may be a contact binary, and/or a merger in which the two stars of the
inner pair coalesce to form a single rapidly rotating star such as
AB$\sim$Dor, a very rapidly  rotating K dwarf that is probably about 50
Myr old.